Low-mass ophthalmic lens

ABSTRACT

The disclosed ophthalmic lens includes a posterior surface and an anterior surface having a spherical central optical correction zone, an aspheric intermediate zone, and a peripheral zone. The intermediate and peripheral zones are not used for optical correction. The posterior surface includes a central optical zone, an intermediate zone, and a peripheral zone. The central optical zone of the anterior surface has a larger diameter than the central optical zone of the posterior surface. Similarly, the intermediate zone of the anterior surface has a larger diameter than the intermediate zone of the posterior surface. The inventive construction is particularly beneficial when applied to a rigid contact lens, particularly rigid contact lenses made using a high DK material.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/377,985 filed Aug. 20, 1999, by Neil R. Hodur et al.entitled “LOW-MASS OPHTHALMIC LENS,” the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention generally pertains to an ophthalmic lens.More particularly, the present invention relates to a contact lens, andeven more particularly relates to a rigid gas-permeable contact lens.

[0003] When contact lenses were first commercialized, they were made ofa rigid material, such as polymethylmethacrylate (PMMA). As shown inFIG. 1, such contact lenses had a posterior surface 10 including acentral optical zone 12 and one or more peripheral zones 14, 16. Opticalzone 12 on posterior surface 10 was defined by a generally sphericalsurface having a radius of curvature corresponding to a measured basecurve of the cornea upon which the contact lens was to be worn. Theperipheral zones (14, 16) were also defined by spherical curves and weretypically flatter than the curve defining central optical zone 12. Theperipheral zones were dimensioned to closely fit the outer area of thecornea. The anterior surface 20 of these early contact lenses typicallyhad one to three zones, including a front central optical correctionzone 22, an optional intermediate zone 24 and an optional peripherallenticular zone 26. Both of these anterior surface zones were defined byspherical curves, with the radius of curvature of anterior centraloptical zone 22 being selected so as to provide the appropriate opticalpower for correction of the patient's hyperopic, myopic, and/orastigmatic condition. The radius of peripheral lenticular zone 26 onanterior surface 20 was selected so as to provide a transition fromcentral optical zone 22 to an edge 17 of the contact lens, whichtypically had a thickness of, for example, 0.125 mm. The transitions 25between zones on anterior surface 20 generally aligned with transitions(18, 28) between zones on posterior surface 10.

[0004] In the late 1960's, contact lenses were modified somewhat byremoving the discrete junctions (18) between the zones by joining thespherical curves with tangential transitions (28). Subsequently, theposterior surfaces of the contact lens were made aspheric so as to moreclosely fit the anterior surface of the cornea, which is also aspheric.

[0005] While soft contact lenses were developed subsequent to rigidlenses and are currently in wide use, such soft contact lenses cannot beused as effectively to correct the vision of individuals having moresevere optical impairment. Thus, there remains a large market for rigidand rigid gas-permeable (RGP) lenses.

[0006] Individuals who wear rigid lenses may experience discomfort,which is typically caused by the sensation of having a rigid foreignobject with a significant amount of mass on the individual's eye. Whileconsideration has been given to reducing the thickness of the rigidcontact lenses so as to reduce their mass and increase oxygenpermeability, thin rigid contact lenses have not been commerciallyfeasible due to the weakening of the contact lens structure and theresultant increased likelihood of breakage, warpage, and flexure.Flexure in a rigid contact lens is undesirable because the lens flexeseach time the patient blinks thereby resulting in variable visioncorrection.

[0007] Thin lens designs gained worldwide popularity with theintroduction of Syntex's Polycon I material in 1979. That material, nowcoined a silicon/acrylate, had an oxygen permeability (DK) of 5×10⁻¹¹.The lenses were available in inventory designs of 9.5/8.4, 9.0/7.8, and8.5/7.3 (outer diameter/optical zone). The 9.5-mm diameter lensincorporated a specific anterior lenticular design and a standardspherical tri-curve posterior design. The lens was fitted approximately1.00 to 1.50 D flatter than “K” to permit lid/lens attachment inunobstructed upward lens movement. “K” is considered to be the flattestcentral curve of the cornea.

[0008] The Polycon I lens was eventually phased out and replaced withthe Polycon II material, which has a DK=12×10⁻¹¹. Past attempts toproduce thin RGP lens designs in moderate to high DK materials have metwith only limited success. Material brittleness, warpage, and base curveinstability as well as visual interference caused by flexure haveprevented the designs from enjoying the success of the low DK PolyconII.

[0009] Therefore, a need has existed in the rigid contact lens marketfor a contact lens having a lower mass while retaining structuralrigidity and strength.

SUMMARY OF THE INVENTION

[0010] Accordingly, it is an aspect of the present invention to solvethe above problems by providing a low-mass rigid contact lens that hasthe same overall rigidity and strength as a conventional rigid contactlens. It is another aspect of the present invention to provide a rigidcontact lens having approximately 50 percent less mass than aconventional rigid contact lens. It is an additional aspect of thepresent invention to provide a rigid contact lens that is much thinnerthan the conventional rigid contact lens and thereby provides foradditional oxygen transmission through the lens. To achieve these andother aspects and advantages, an ophthalmic lens according to thepresent invention comprises a posterior surface and an anterior surface,where the anterior surface has a spherical central optical correctionzone, an aspheric intermediate zone, and an aspheric peripheral zone.The aspheric intermediate and peripheral zones are not used for opticalcorrection, while the central optical correction zone is used forproviding correction at a single focal length.

[0011] These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the drawings:

[0013]FIG. 1 is a cross-sectional view of a portion of a conventionalrigid contact lens;

[0014]FIG. 2 is a cross-sectional view of a portion of a contact lensconstructed in accordance with the present invention; and

[0015]FIG. 3 is another cross-sectional view of a contact lensconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] As discussed above, the present invention pertains to anophthalmic lens, and preferably to rigid and RGP contact lenses. Whilethe invention is described below with reference to RGP lenses, theinvention is more broadly applicable to any ophthalmic lens. As used anddescribed herein, an “ophthalmic lens” is any lens pertaining to theeye, which includes, but is not limited to, spectacle lenses,intraocular lenses, contact lenses, and the cornea itself.

[0017] A contact lens 100 constructed in accordance with the presentinvention is shown in FIG. 2. As illustrated, contact lens 100 includesa posterior surface 110 and an anterior surface 120. Anterior surface120 has a spherical central optical correction zone 122, an asphericintermediate zone 124, and a peripheral zone 126. Zones 122, 124, and126 are preferably seamlessly transitioned in the transition regionsreferenced with arrows 125. As further described below, peripheral zone126 is also preferably aspheric.

[0018] Posterior surface 110 of contact lens 100 includes a centraloptical zone 112, an intermediate zone 114, and a peripheral zone 116.As further described below, the transitions 125 between zones on theanterior surface 120 are offset from transitions 118 of posteriorsurface 110 so as to be approximately halfway between transitions 118.Thus, central optical zone 122 of anterior surface 120 has a largerdiameter than central optical zone 112 of posterior surface 110, andintermediate zone 124 of anterior surface 120 has a larger diameter thanintermediate zone 114 of posterior surface 110.

[0019] The posterior surface of contact lens 100 is known as the fittingsurface because it must align with the topography of the anteriorsurface of the cornea, resting on the pre-corneal tear layer, yetprovide an adequate amount of tear to circulate underneath the lens toprovide for proper corneal physiology and cooling as well as ridding ofwaste products from metabolism. The anterior surface is known as thepower or optical surface because it is generally created to provide thenecessary blending of light to satisfy the patient's visual needs. Theinventive contact lens was constructed to be thin, yet not flex on thecorneal surface. The inventive contact lens utilizes a new approach tothe design of such contact lenses insofar as it utilizes aspheric curveson the anterior surface to control the thickness and geometric form of acontact lens. The posterior surface of contact lens 100 can be sphericalor aspherical, but zones 24 and 26 utilize continuous aspherical curvesto control the form of the lens, not the optics. The aspheric curves onanterior surface 120 are not optical in nature. The optics of the lensis through the spherical center optical zones 110 and 120. This allowsfor clean optics at a distance.

[0020] Incorporating this new design approach with high DK materials hasresulted in a thin RGP lens having approximately 48 percent less massthan prior RGP lenses, with a firm optical portion that will resistflexure. While this design approach utilizes front surface aspherics tomaintain a controlled thickness, it also provides for the requiredstructural integrity in resistance to flexure by utilizing staggeredtransition zones on the anterior and posterior surfaces. Specifically,by locating the transitions 125 between zones on anterior surface 120,such that they are halfway between transitions 118 on posterior surface110, a “dome” effect is created. The dome effect ensures uniformstrength throughout the lens and thus allows one curve to supportanother. Absent this staggering of transition zones, a thin contact lenswould not exhibit the dome effect and would not have sufficientstructural rigidity to be resistant to flexure, breakage, or warpageunless the thickness was increased.

[0021] While the idea of using aspheric surfaces on either the posterioror anterior surfaces is not by itself novel, lenses that utilizedaspheric anterior surfaces are multi-focal lenses that utilize theaspheric surface for continuous correction for all distances fromreading to infinity. Such aspheric curves, however, were not previouslyutilized on the anterior surface of a contact lens providing a singlezone of optical correction (i.e., lenses designed to correct eitherhyperopia or myopia), and particularly were not used only in non-opticalcorrecting zones of the contact lens. As explained further below, theinventive contact lens utilizes aspheric anterior curves that may haveeccentricities opposite the aspheric curves used in multi-faced designs.

[0022] Contact lens 100 was preferably made of a homogenous rigidgas-permeable material. While contact lens 100 may be made of any suchmaterial, lens 100 is preferably made of a material having oxygenpermeability (DK) of at least 28×10⁻¹¹. As used herein, “oxygenpermeability” (DK) of a lens material is the rate at which oxygen willpass through a material. Oxygen permeability is conveniently expressedin units of barrers, where “barrer” is defined as [(cm³oxygen)(mm)/(cm²)(sec)(mm Hg)]×10⁻¹⁰. Examples of preferred materialsinclude Boston EO™ from Polymer Technology of Rochester, N.Y.(DK=44×10⁻¹¹) and Fluoroperm® 30 and HDS™ from Paragon Optical Companyof Tempe, Ariz. The most preferred material being Boston ES® fromPolymer Technology (DK=28×10⁻¹¹).

[0023] Central optical zone 112 on posterior surface 110 is defined by abase curve. As will be apparent to those skilled in the art, the curve,although two-dimensional, may be used to define a surface of rotation,since the contact lens is symmetric about its central axis 130. The basecurve defining central optical zone 112 corresponds to the base curvemeasured on the anterior surface of the cornea. Central optical zone 112may be sperhical or aspherical. Intermediate zone 114 on posteriorsurface 110 is defined by a fitting curve, which is flatter than thebase curve. The radius r_(P2) of the fitting curve is determined as afunction of the base curve (r_(base)) as explained in more detail below.Peripheral zone 116 on posterior surface 110 has a radius r_(P3) that isrelated to and flatter than the fitting curve. The degree to which thefitting curve is flatter than the base curve is dependent upon thediameter of the lens. Table 1 below provides an example of therelationships between the base curve, fitting curve, vault curve, anddiameter of lens for lenses of four different diameters. TABLE 1DIAMETER OF LENS, Y_(T) FITTING CURVE, r_(P2) VAULT CURVE, r_(p3) 8.5 mmbase curve(r_(base)) + 1.00 mm fitting curve(r_(P2)) + 1.50 mm 9.0 mmbase curve(r_(base)) + 1.10 mm fitting curve(r_(P2)) + 1.50 mm 9.2 mmbase curve(r_(base)) + 1.20 mm fitting curve(r_(P2)) + 1.50 mm 9.5 mmbase curve(r_(base)) + 1.20 mm fitting curve(r_(P2)) + 1.50 mm

[0024] The diameters of the optical zones on the posterior surface 110are determined as a function of the diameter of the lens. As shown inFIG. 3, the central optical zone 112 has a diameter of Y_(P1) andintermediate zone 114 is annular in shape and has an inner diameter ofY_(P1) and an outer diameter of Y_(P2). Peripheral zone 116 is alsoannular in shape and has an inner diameter corresponding to the outerdiameter of zone 114 (i.e., Y_(P2)), and an outer diameter correspondingto the total diameter of the lens Y_(T). On anterior surface 120,central optical zone 122 has a diameter of Y_(A1). Intermediate zone 124is annular in shape and has an inner diameter of Y_(A1) and an outerdiameter of Y_(A2). Peripheral zone 126 is also annular in shape and hasan inner diameter of Y_(A2) and an outer diameter corresponding to thetotal diameter Y_(T) of the lens. Table 2 below defines these diametersfor each of the four total diameters Y_(T) listed in Table 1 above.TABLE 2 First Second Diameter Posterior Posterior of Transition,Transition, First Anterior Second Anterior Lens, Y_(T) Y_(P1) Y_(P2)Transition, Y_(A1) Junction, Y_(A2) 8.5 mm 7.3 mm 8.1 mm 7.7 mm 8.3 mm9.0 mm 7.8 mm 8.4 mm 8.1 mm 8.6 mm 9.2 mm 8.0 mm 8.8 mm 8.4 mm 9.0 mm9.5 mm 8.3 mm 9.1 mm 8.7 mm 9.3 mm

[0025] As apparent from viewing FIG. 3 and Table 2 above, thetransitions 125 are midway between transitions 118 on posterior surface110. Again, this provides the dome effect mentioned above, whichincreases the structural rigidity of the lens.

[0026] The three curves defining the three zones on anterior surface 120are designed to minimize the overall thickness of the lens, allowingcontrol of the lens thickness from edge to edge. The curve definingcentral optical zone 122 is configured to obtain the optics necessary toprovide the prescription needs of the lens. The curve definingintermediate zone 124, also referred to as the lenticular control curve,is aspheric in nature and is mathematically determined to keep the lensat substantially a constant thickness. The third anterior curve,referred to as the edge control curve, defines peripheral zone 126. Theedge control curve is designed to provide an edge thickness of 0.125 mmafter finishing of the lens. The 0.125 mm thickness at the edge isgenerally known as an optimum edge thickness for the contact lens.

[0027] Given that the base curve radius r_(base) and the lens diameterY_(T) determine the radius of curvatures of each of the curves definingthe optical zones on posterior surface 110 and given that the diameterof the lens Y_(T) determines the location of the transitions of theseposterior zones, an inventory of lens blanks may be kept in stock by alens manufacturer that have different base curves for the differentdiameter lenses without regard to the optical power that is subsequentlyprovided by turning and polishing the anterior surface of the lens blanktypically with a computerized numerically controlled (CNC) lathe. Thus,the inventive contact lens design enables a relatively low number ofdifferent types of lens blanks to be kept in inventory. The lens blanksmay be preconstructed as piano lenses having no optical power or not bepre-cut. Thus, the anterior surface, particularly the surface definingthe central optical zone, has a radius of curvature that correspondsclosely to the base curve of the lens blank. The manner in which suchlens blanks are then turned to provide the requisite optical power perthe patient's prescription is described below.

[0028] The optical power Fv′ (i.e., dioptrics) of the lens wasdetermined from the following formulas: $\begin{matrix}{{Fv}^{\prime} = {\frac{F_{1}}{1 - {cF}_{1}} + F_{2}}} & (1) \\{F_{1} = \frac{n_{lens} - n_{air}}{r_{A1}}} & (2) \\{F_{2} = \frac{n_{air} - n_{lens}}{r_{base}}} & (3) \\{c = \frac{x}{n_{lens}}} & (4)\end{matrix}$

[0029] Where r_(base) is the base curve radius defining central opticalzone 112 of posterior surface 110, r_(A1) is the radius of curvature ofthe curve that defines central optical zone 122 of anterior surface 120so as to impart the needed optical power, x is the thickness inmillimeters of the lens at a particular y value which is the distancefrom the central optical axis of the lens, n_(lens) is the refractiveindex of the lens material, n_(air) is the refractive index of air, F₁is the refractive power of an anterior optical zone surface 122, F₂ isthe refractive power of posterior optical zone surface 122, and c is aconvergence factor. The lens thickness is taken into account bymeasuring various Sagitta (s) for various values of y, while ensuringthat the thickness x does not vary by more than a target curve depthdifferential of 0.08 mm. The target curve depth differential may varywith different lens materials having different degrees of structuralrigidity. For example, if a material having a greater structuralrigidity were used, the target curve depth differential could be loweredto 0.04 mm for instance. The Sagitta are determined by the equationbelow:

s=r−{square root}{square root over (r²−y₂)}  (5)

[0030] The best conic_section to create the curve necessary on the frontto give the desired thickness may be derived from eccentricity values,which may be expressed as an e value or a p value. The e and p valuesare derived from the equations below: $\begin{matrix}{p = \frac{{2{rx}} - y^{2}}{x^{2}}} & (6)\end{matrix}$

 p=1−e ²  (7)

[0031] The p represents the amount of asphericity based upon a conicalsection, where p>1.00 is defined as a prolate ellipse, 0<p<1 is definedas an oblate ellipse, p=0 is defined as a parabola, and p<0 is definedas a hyperbola. The e value is the eccentricity of the curve or theamount it differs from a spherical surface, where e=0 defines a sphere,0<e<1 defines an ellipse, e=1 defines a parabola, and e>1 defines ahyperbola.

[0032] To illustrate how the mathematics above operate, an example isprovided below for a contact lens having a total diameter Y_(T) of 9.2mm, where the optical power is −1.00 diopter and the base curve is setat 7.34. These values would be obtained from the optometrist orophthalmologist writing the prescription for the patient.

[0033] Using Table 1 above, the radius of curvatures r_(P2) and r_(P3)of the curves defining intermediate zone 114 and peripheral zone 116 maybe determined. Specifically, for a 9.2 mm lens having a r_(base)=7.34 mmbase curve, the radius of curvature r_(P2) of the fitting curve definingintermediate zone 114 is 8.54 mm (7.34+1.2) and the vaulting curvedefining peripheral zone 116 has a radius r_(P3)=10.04 mm (8.54+1.5). Wealso know using Table 2 that the diameter Y_(P1) of optical zone 112 onposterior surface 110 is 8.0 mm, the intermediate optical zone outsidediameter Y_(P2) is 8.8 mm, and the outer diameter Y_(P3) of peripheralzone 116 is 9.2 mm. We also know that the diameter Y_(A1) of centraloptical zone 122 on anterior surface 120 is 8.4 mm, the intermediatezone 124 has an outside diameter Y_(A2) of 9.0 mm, and peripheral zone126 has an outside diameter Y_(A2) of 9.2 mm. We also know that therefractive index of air n_(air) is 1.00, and the refractive index of thelens material n_(lens) is 1.45 (for Boston ES® material). Insofar as weknow the base curvature r_(base) is 7.34 mm, we can compute theposterior surface optical power F₂ using Equation 3 above. The opticalpower of this surface is F₂=−61.31. Because the starting lens is a lensblank having plano power (i.e., F_(v)′=0), by setting a target thicknessx for the center of the lens at 0.08 mm, we can determine the anteriorsurface optical power F₁ using Equation 1 above. Then, using Equation 2,we can use this optical power to solve for the initial radius r_(A1) ofthe anterior surface in the central optical zone. Using this approach,we can thus determine that the radius r_(A1) is equal to 7.37 mm. Thenext step is to determine the Sagitta s₁ using radius R_(A1) at a yvalue of 4.0, which corresponds to the first junction point Y_(P1) onthe posterior surface. This Sagitta value is computed using Equation 5above to arrive at a value of s₁=1.18 in this particular example.

[0034] Next, we determine the Sagitta s₂ at a value y=4.4 (correspondingto Y_(P2)) on the fitting curve 114, which has a radius of 8.54 mm.Again, using Equation 5, we find that s₂ is equal to 1.22 at y=4.4. Weuse this same Sagitta S₂ at y=4.2 (corresponding to Y_(A1)) to solve fora radius r_(A2) using Equation 5. This gives us a radius of curvature ofr_(A2)=7.84 mm (for piano power). Using a similar approach, the radiusof curvature r_(A3) of peripheral zone 126 may be found to be 8.65 mm.

[0035] For a lens requiring an optical power of −1.00 diopters(R_(v)′=−1.00), one can then determine the curve profiles required todefine the three zones on anterior surface 120 (the surfaces on theposterior surface 110 remain the same). Because the curvature of zone112 of posterior surface 110 does not change, the optical power F₂ ofthis surface remains equal to −61.31. Thus, using a value R_(V)′=−1.00,F₂=−61.31, a value n_(lens)=1.45, and a value x=0.08 mm, we can computethe refractive power F₁ of anterior surface zone 122 that is requiredusing Equation 1 above. Using this equation, we find that F₁ is equal to60.09. Then, using Equation 2, we compute the required radius ofcurvature r_(A1)′, which is equal to 7.48 mm. Using r_(A1)′, we computea Sagitta S₃ at y=4.2 (corresponding to Y_(A1)), which yields a value of1.29. We then compute a value d, by determining the difference betweenthe radius of curvature r_(A1)′ of the central optical zone 122, whichprovides in combination with zone 112 the required −1.00 diopter opticalpower, and the radius of curvature r_(A2) of the initial intermediatezone 124 of the lens blank. This yields a value d₁=0.36 (7.84-7.48). Wethen obtain a value x at y=4.2 by subtracting the computed d₁ value fromthe s₃ value, which yields x=0.93. Using x=0.93 mm, r=r_(A1)′=7.48 mm,and y=4.2 mm, the p value at y=4.2 may be computed using Equation 6above, which yields a value of p=−4.31. Using Equation 7, theeccentricity value may be computed as e=2.30. The eccentricity value maythus be computed for each point along the curve defining zone 124 usingr_(A1)′=7.48 mm and d₁=0.36 while recomputing a new Sagitta at each yvalue and using that new Sagitta to compute a new x value to plug intoEquations 6 and 7. This sets the front and back curve differential to a0.08 mm thickness value.

[0036] Next, a second value d₂ may be computed by determining thedifference between the front surface central curve radius r_(A1)′ andthe initial radius of curvature r_(A3). This yields a value of d₂=1.17(8.65-7.48). A Sagitta S₄ may be obtained at y=4.5 on anterior surfacezone 126, which yields a Sagitta value of S₄=1.51. We can then subtractthe d₂ value from this Sagitta S₄ to obtain an x value of 0.34 mm. Theabove Sagitta S₄ is obtained using a radius of curvature of 7.48 mm aty=4.5 (corresponding to Y_(A2)). Then using a value r=7.48 mm, x=0.34mm, and y=4.5 mm, the p value (at y=4.5) can be computed using Equation6 above, which yields a value of −131.17 for this particular example.The eccentricity value of e is equal to 11.50. This sets the front andback curve differential to a 0.08 mm thickness value. Again, theeccentricity values for each point along the curve defining zone 126 maybe determined in this manner.

[0037] As will be appreciated by those skilled in the art, otherdioptric powers may be obtained using the same lens blank. The surfacesmay thus be defined for such lenses by setting R_(v)′ equal to thedesired dioptric power and running through the equations as performed inthe above example. For example, for a lens requiring an optical power of−7.00 diopters (F_(v)′=−7.00) and having the same size (9.2 mm) and thesame base curve (7.34 mm), one can use the same lens blank as the lensdescribed above having −1.00 dioptric power. One can then determine thecurve profiles required to define the three zones on anterior surface120 (the surfaces on the posterior surface 110 remain the same). Becausethe curvature of zone 112 of posterior surface 110 does not change, theoptical power F₂ of this surface remains equal to −61.31. Thus, usingR_(V)′=−7.00, F₂=−61.31, n_(lens)=1.45, and x=0.08 mm, we can computethe focal power F₁ of anterior surface 122 that is required usingEquation 1 above. Using this equation, we find that F₁ is equal to54.13. Then, using Equation 2, we compute that r_(A1)′ is equal to 8.31mm. Using r_(A1)′, we compute a Sagitta at y=4.2, which yields a valueof 1.14. We then compute a value d₁ by determining the differencebetween the radius of curvature r_(A1)′ of the central optical zone 122and the radius of curvature r_(A2) of the initial intermediate zone 124of the lens blank. This yields a value d₁=0.47 (8.31-7.84). We thenobtain our value x at 4.2 by subtracting the computed d₁ value from thes value of 1.14, which yields x=0.67. Using x=0.67 mm, r=r_(A1)=8.31 mm,and y=4.2 mm, the p value at y=4.2 may be computed using Equation 6above, which yields a value of p=−14.49 at y=4.2. Using Equation 7, theeccentricity value may be computed as e=3.94. This sets the front andback curve differential to a 0.08 mm thickness value.

[0038] Next, a second d₂ value may be computed by determining thedifference between the front surface central curve radius r_(A1)′ andthe initial radius of curvature r_(A3). This yields a value of d₂=0.34(8.65-8.31). A Sagitta may be obtained at y=4.5 on anterior surface zone126, which yields a Sagitta value of 1.32. We can then subtract the d₂value from this Sagitta value to obtain an x value of 0.98 mm. The aboveSagitta value is obtained using a radius of curvature of 8.31 at y=4.5.Then using a value r=8.31 mm, x=0.98 mm, and y=4.5 mm, the p value canbe computed using Equation 6 above, which yields a value of −4.13 aty=4.5 for this particular example. The eccentricity value of e is equalto 2.26. This sets the front and back curve differential to a 0.08 mmthickness value.

[0039] While the above invention has been described with respect torigid contact lenses, it will be appreciated by those skilled in the artthat the inventive concepts may also be applied to other ophthalmiclenses, such as intraocular lenses, spectacle lenses, and soft contactlenses. Additionally, the inventive concepts described above may beemployed in constructing other optical lenses, where the total thicknessis to be controlled. Further, this technology can be applied to laserrefractive surgery, where the cornea is ablated to reshape the corneaand thereby provide optical correction. By reshaping the peripheralcornea as an aspheric surface rather than a spherical surface (as ispresently performed), less of the cornea may need to be ablated whilestill providing the necessary optical correction.

[0040] The above description is considered that of the preferredembodiments only. Modifications of the invention will occur to thoseskilled in the art and to those who make or use the invention.Therefore, it is understood that the embodiments shown in the drawingsand described above are merely for illustrative purposes and notintended to limit the scope of the invention, which is defined by thefollowing claims as interpreted according to the principles of patentlaw, including the doctrine of equivalents.

The invention claimed is:
 1. An ophthalmic lens comprising a posteriorsurface and an anterior surface, said anterior surface having aspherical central optical correction zone, an aspheric intermediatezone, and a peripheral zone, wherein said intermediate and peripheralzones are not used for optical correction.